Add a comment explaining why the OHCI mapping has been commented out.

sys/arm/at91/kb920x_machdep.c

@@ -159,6 +159,15 @@ static const struct pmap_devmap kb920x_devmap[] = {
 		VM_PROT_READ|VM_PROT_WRITE,                             
 		PTE_NOCACHE,
 	},
+	/*
+	 * We can't just map the OHCI registers VA == PA, because
+	 * AT91RM92_OHCI_BASE belongs to the userland address space.
+	 * We could just choose a different virtual address, but a better
+	 * solution would probably be to just use pmap_mapdev() to allocate
+	 * KVA, as we don't need the OHCI controller before the vm
+	 * initialization is done. However, the AT91 resource allocation
+	 * system doesn't know how to use pmap_mapdev() yet.
+	 */
 #if 0
 	{
 		/*